Cathodochromic image screen and method for preparing cathodochromic sodalite for said image screen

ABSTRACT

An image screen of a dark trace cathode ray tube comprises a substrate having a novel sodalite cathodochromic layer thereon. The novel sodalite is characterized in that it is spherically shaped, porous and contains a sodium halide deficiency of 10 - 20 weight percent. The novel sodalite is prepared by a hydrothermal growth technique employing excess sodium hydroxide and sodium halide and wherein the reaction vessel is heated to a uniform temperature and rapidly cooled to prevent growth of dedcahedral crystals.

iliiiied States Patent Shidlovsky Nov. 20, 1973 [54] CATHODOCHROMIC IMAGE SCREEN AND 2,504,674 4/1950 Fonda 117/33.5 C METHOD FOR PREPARING 2,761,846 9/1956 Medved.. 252/30l.4 F 3,598,750 8/1971 Phillips l17/33.5 C

CATHODOCHROMIC SODALITE FOR SAID IMAGE SCREEN Igal Shidlovsky, Princeton, NJ.

Assignee: RCA Corporation, New York, N.Y.

Filed: May 27, 1971 Appl. No.: 147,392

[75] Inventor:

[52] U.S. Cl. l17/33.5 C, 106/288 B, l17/33.5 CM, 117/335 CP,117/33.5 CS, 161/191,

in U

H II

Primary ExaminerWilliam D. Martin Assistant ExaminerWilliam R. Trenox Att0rneyG. H. Bruestle [57 ABSTRACT 5 Claims, 3 Drawing Figures Patented Nov. 20, 1973 3,773,540

2 Sheets-Sheet 1 ATTORNEY Patented Nov. 20, 1973 3,773,540

2 Sheets-Sheet 2 CATHODOCHROMIC IMAGE SCREEN AND METHOD FOR PREPARING CATHODOCHROMIC SODALITE FOR SAID IMAGE SCREEN BACKGROUND OF THE INVENTION This invention relates to an improved cathodochromic image screen of a dark trace cathode ray tube and more particularly to an improved sodalite material useful in such screens and the method of preparation of this sodalite.

Sodalite is known to be a photochromic material as illustrated in U. S. Pat. No. 2,761,846 issued to David B. Medved. In addition, the use of sodalite as a cathodochromic material in dark trace ray tubes is described in US. Pat. No. 2,752,521 issued to Henry F. lvey. The term photochromic materials as used herein refers to materials which can be reversibly switched from one absorption state to another absorption state by means of light. The term cathodochromic material as used herein refers to materials which can be colored by means of electron beam bombardment, which coloration can be removed by means of heat.

An important feature of any cathodochromic device such as a dark trace cathode ray tube is the contrast ratio and gray scale attainable. It is, of course, preferred to have as high a maximum obtainable contrast ratio as possible.

In the past, sodalite has been prepared mainly in an effort to enhance its photochromic properties and there was no realization of the fact that requirements for preparing sodalite having a high contrast ratio for use as a cathodochromic material are different from the requirements for the preparation of sodalite useful as a photochromic material. Prior art sodalite typically exhibited the dodecahedral crystal structure when examined under an electron microscope. Dark trace cathode ray tubes employing sodalite prepared by prior art techniques typically have maximum cathodochromic contrast ratios in the range of about 3:1 to 7:1. This limit in contrast ratio also limits the gray scale attainable in these tubes. I have discovered a method for preparing sodalite useful in a cathodochromic image screen, which has a cathodochrmoic contrast ratio at saturation in the neighborhood of at least to l and as high as 50 to 1. While the cathodochromic contrast ratio of this sodalite is high, the photochromic contrast ratio attainable in this material is low, for example, in the neighborhood of 3 to 1. In addition, the novel sodalite has unique physical characteristics.

SUMMARY OF THE INVENTION A cathodochromic image screen comprises a substrate having a layer of cathodochromic sodalite thereon. The cathodochromic sodalite is characterized by the fact that as observed under an electron microscope the sodalite particles are essentially spherically shaped having a fairly uniform diameter.

The novel spherically shaped sodalite particles are prepared by a novel hydrothermal crystal growth technique The novel hydrothermal growth technique comprises the steps of preparing an aqueous growth mixture comprising aluminum oxide, silicon dioxide, sodium hydroxide and a sodium halide. The aluminum oxide and silicon dioxide are in stoichiometric proportions while the sodium hydroxide and sodium halide are in excess of stoichiometric ratios. This growth mixture or slurry is sealed in a hydrothermal reaction bomb.

The reaction bomb is heated to the desired temperature, which temperature is a function of the concentration of the materials in the slurry and the reaction pres sure. The temperature employed is uniform over the entire reaction bomb and is maintained for an extended period of time. The bomb is finally cooled to room temperature at a relatively rapid cooling rate so as to prevent the formation of dodecahedral crystals.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a cathode ray tube having a cathodochromic sodalite image screen.

FIG. 2 is a photo-micrograph showing sodalite of the prior art type grown by prior art hydrothermal growth technique.

FIG. 3 is a photo-micrograph showing the novel cathodochromic sodalite prepared by the novel hydrothermal technique described herein.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a cathode ray tube 10 having a cathodochromic image screen 12. The cathodochromic image screen 12 comprises a layer 14 of a novel cathodochromic sodalite material supported on an optically transparent faceplate 16. The cathodochromic layer 14 may be formed on the faceplate 16 by well-known, conventional methods such as settling and spraying. These techniques are well known in the cathode ray tube art for the preparation of phosphorus screens. The layer 14 of cathodochromic material is enclosed in an evacuated envelope 18 containing an electron gun 20 and deflection means 22.

The cathodochromic sodalite comprising the novel image screen differs greatly in physical appearance from prior art sodalite material. A photo-micrograph taken by electron scan microscopy of a typical prior art type photochromic sodalite grown by the conventional hydrothermal method is shown in FIG. 2. The dodecahedral crystal faces of the sodalite are readily apparent from the photo-micrograph and the crystal size of the individual crystallites range from about 6 to 20 microns. In comparison, as shown in FIG. 3 by a photomicrograph taken of sodalite included in the novel image screen, the difference between this sodalite and the prior art sodalite is apparent. The sodalite incorporated in the novel screen is spherical in nature and does not show dodecahedral crystal planes. In addition, many of these spheres are hollow and the particle size is much more uniform. Typically, the particle size is less than about 7 microns and generally around 5 microns with percent of the particles being from 4 to 6 microns in diameter. The prior art sodalite shown in FIG. 2 was grown in accordance with the usual prior art technique of providing a temperature gradient in a hydrothermal bomb to allow for transport of materials and employing slow cooling, both of which lead to dodecahedral crystal growth. In comparison, the sodalite shown in FIG. 3 was grown hydrothermally in a reaction bomb kept as a uniform temperature, that is, without a temperature gradient present, and employing quick cooling of the bomb. This process is discussed in more detail below.

Generally, prior art photochromic sodalites have a general formula Na Al Si O '2NaX where X is Cl, Br" and I. These materials have a well defined crystal structure as aforesaid. Also, the sodalite crystal structure can be maintained by completely or partially replacing the halogen with OH or by partially replacing the halogen by a Group Vl element such as oxygen, sulphur, selenium or tellurium with the appropriate stoichiometric corrections in order to maintain charge neutrality. Prior art sodalites contain an electron donor impurity such as the Group VI element or iron and these impurities are involved in the coloration mechanism. The density of these crystallized prior art sodalites is high and approaches theorectical density of the material.

In comparison, it has been discovered that impurities are not necessary for the cathodochromic processes and that different mechanisms are involved in the coloration process of cathodochromic sodalites. Cathodochromic sodalites are sensitized by the removal of alkali halide molecules from the sodalite structure creating crystal defects which can form F centers. The removal of the alkali halide molecules is accomplished by heating the sodalite material during the sensitization process. These alkali halide molecules are diffused from inside the crystals to the surfaces where they evaporate. When the material is in the form of a well defined dense crystal as in the prior art photochromic sodalites, diffusion is slow and the losses of alkali halides are limited, thereby reducing the cathodochromic sensitivity of the resulting material. I have found that in order to have a good cathodochromic material, the sodalite particles should be very poorly crystalized and should preferably be porous. Such material allows for a more rapid and complete diffusion and evaporation of the alkali halide from the crystals and results in a cathodochromic sodalite which has a high cathodochromic sensitivity, a high contrast ratio and an increased penetration depth of the electron beam for coloration.

I have discovered a hydrothermal method for preparing such poorly crystallized porous sodalite material. In this process, the starting materials, which consist of a water slurry of aluminum oxide, silicon dioxide, sodium hydroxide and sodium halide is placed in a reaction bomb which is exposed to combinations of elevated temperature and high pressure. The aluminum oxide and silicon dioxide in the slurry are present in stoichiometric proportions. The complexing materials, that is, the sodium hydroxide and alkali halide, are present in excess of stoichiometric quantities. Unlike prior art hydrothermal growth techniques, here the reaction bomb is maintained at a uniform temperature without a gradient along the reaction vessel so that no transport will occur. Also, after heating the material under high pressure to allow for dissolution of the aluminum oxide and silicon dioxide in the form of complex anions and cations, heating is stopped abruptly and the reaction vessel is cooled to room temperature so that the material which settles from the solution due to the reduction in solubility does not crystallize but instead settles in the form of small spherical or ball shaped particles.

EXAMPLE I 5.140 gm. of NaBr, 5.994 gm. of NaOH, 7.560 gm. A1 9.000 gm. SiO and 35 mgs. of FeCl are mixed together with about 150 cc. of water to form a slurry. The slurry is heated while stirring to evaporate water to the desired volume. The slurry is then-transferred to the reaction tube and selaed. In this particular example, the reaction tube has a volume of 70 cc. and is filled to 70 percent of capacity. The supernatant solution of the slurry contains NaOH at a concentration of about 3N and NaBr at a concentration of about 0.3N. The reaction tube is heated to a temperature of 385 C for 7 days. After the 7 day period, the reaction tube is cooled at a rate of about 10 20 C per minute to a temperature of 200 C and then to room temperature over an additional 2 hour period. The reaction tube is then opened and the solid material therein is washed with water to remove excess sodium hydroxide and sodium halide. The washed material, which is sodalite, is dried and heat treated for 30 minutes at 900 C under a hydrogen atmosphere for sensitization. During this period, 12 percent of the sodium halide is removed from the sodalite. The resulting material is small ball shaped particles having sizes up to about 5 6 microns. Settled slides containing 3 mg of the novel sodalite per cm in a potassium silicate potassium sulfate binder were tested under 25 KV electron beam radiation. The sensitization of these slides was high as only 400 /.C/in of radiation was required to achieve a contrast ratio of 10:1. The contrast ratio of these slides upon saturation was about 50:].

The exact amount of starting materials employed will vary according to the size and percent of filling of the reaction tube and to the amount of excess sodium hydroxide and sodium halide desired. Since the solubility of sodalite in the solution is a function of the temperature, pressure, and concentration of complexing agents, the specific temperature to which the reaction vessel will be heated will also vary. In addition, the heating time will vary with the particular amounts of starting materials and the reaction tube size. In general, the reaction tube must be heated to above the critical temperature, (about 356 C) which is required for the formation of sodalite. Typical temperature to which the reaction tube is heated is about 400 C for a period of from about 5 15 days depending upon the size of the batch. Typical pressures present in the reaction vessel are from 10,000 15,000 psi. Generally, the starting materials contain sodium hydroxide and sodium halide in an excess of the stoichiometrically required quantities so as to result in above about 2N NaOH and above about 0.1N NaX in the supernatant liquid of the slurry.

Although sensitization is shown to be accomplished in a hydrogen atmosphere, this step may be carried out in nitrogen and oxygen or other gas which is nonreactive with sodalite. Generally, sensitization is accomplished by heating the sodalite at a temperature of 900 C or greater for a time sufficient to allow from about 10 20 percent weight loss of the alkali halide of the sodalite. This weight loss is due to evaporation of the sodium halide from the sodalite; hence causing sodium and halogen vacancies in the sodalite crystals.

EXAMPLE II 10. gm. SiO 8.840 gm. Al O 13.320 gm. NaOH, 10.809 gm. Nal are placed in about 150 cc. of water to form a slurry. The slurry is heated while stirring to evaporate the water to the desired volume. The slurry is then transferred to the reaction tube and sealed. In this particular example, the reaction tube has a volume of 78 cc. and is filled to percent of capacity. The supernatant solution of the slurry contains NaOH in a concentration of 3.0M and NaI in a concentration of 0.3N. The sealed reaction tube containing the slurry is heated at a uniform temperature of 370 C for 9 days. The calculated pressure in the tube is about 10,500 psi. After the ninth day, the heat is removed and the reaction vessel is cooled to room temperature in a period of about 2 /2 hours. The reaction tube is then opened and the solid material therein is washed with water to remove excess NaBr and NaOH. The washed material is sodalite of the iodide type. The material is dried and sensitized by heating for 45 minutes at 900 C in hydrogen. The sodalite undergoes a percent weight loss of the Nal during this time period. Settled slides prepared from this sodalite and tested in the same manner as indicated with respect to Example I, achieved a contrast ratio of 10:1 after irradiation of 600 pC/in". The saturation contrast ratio of this material is in excess of :1.

EXAMPLE lII Following the same general procedure as set forth in the previous examples, the following quantities of material were prepared and tested under the conditions as set forth below:

2.770 gm. NaCl 9.450 gm. NaOH 5.724 gm. A1 0 6.725 gm. SiO 70 percent filling (solution 50 cc) to make a 0.11N NaCl and a 2.5N NaOH solution Temp. 400 C Pressure 10,000 psi Reaction time 8 days Cooling time 150 min to room temperature Sensitization 90 min H2 900C Loss of 8.5 percent of the NaCl Same procedure as above.

This sodalite was tested under a 20,000V source of electrons on a loose powder sample. A contrast ratio of 7:] was achieved with irradiation of 3,000 ;1.C/in

EXAMPLE IV Following the same procedure as set forth in the previous examples, the following quantities of material were prepared and tested under conditions as set forth below:

3.508 gm. NaCl [0,821 gmv NaOH 8.500 gm. SiO 6 5 filling to make a solution which is 3N NaOH and 0.3N NaCl Temp. 400

Pressure 15,000 psi Time 14 days Cooling time l50 min to room temperature Sensitization min H 900C Loss 9 percent of the NaCl This sodalite was tested under a 20,000 V source of electrons on a loose powder sample. A contrast ratio of 10:] was achieved with irradiation of 1,600 pC/in The saturation contrast ratio of this material was greater than 20:1.

What I claim is:

1. An image screen of dark trace cathode ray tube comprising a substrate having a layer of cathodocrhomic sodalite particles thereon, said sodalite particles characterized in that they are essentially spherically shaped and porous having a particle size of less than 7 microns, and wherein said image screen has a saturation cathodoehromic contrast ratio of at least 20:1.

2. The image screen recited in claim 1 wherein said sodalite has a sodium halide deficiency of from 10 20 weight percent.

3. The image screen recited in claim 1 wherein said sodalite particles have a sodium halide deficiency from stoichiometry of from 10 20 weight percent and wherein said image screen has a saturation cathodochromic contrast ratio of at least 20:1.

4. The image screen recited in claim 3 wherein at least 90 percent of said sodalite particles have a particle size of from 4 6 microns in diameter.

5. In a dark trace cathode ray tube, an image screen comprising a substrate having a layer of cathodochromic sodalite particles thereon of the type wherein the halogen ion is a bromide ion, said sodalite characterized in that the particles are spherical in shape having an average particle size of from 4 6 microns in diameter and contain less than the stoichiometric quantity of NaBr, said cathode ray tube having a cathodoehromic contrast ratio at saturation of at least 20:1.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3,773,540 Dated November 20. 1973 Inventor(s) I gal Shidlovskv It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, change "dedcahedral" to dodecahedral Column '1, line 59, between "nique" and "The" insert a period Column 2 line 59, change "as" to at Column 3, line 64, change "selaed" to sealed Column 5, line 43, change "10,821" to 10.821

On the cover sheet, '5 Claim? Should read L; Claims Column 6, Claims *h and 5"should read 3. and L Colman 0, line 31, "3" should read l Signed and sealed this 21st day of May 197A.

Attest:

EDNII-ID LLFLETSHJJEQJLZ, v J. IUIRSIIALL DAt-III Attesting Officer Commissioner of Patents FORM P0-1050 (IO- uscoMM-Dc suave-P09 Q U.S. GOVERNMENT PRINTING OFFICE l9! 0-366-334 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,773,540 Dat November 20. 1973 Inventor(s) I gal Shidlovskx It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, change "dedcahedral" to dodecahedral Column 1, line 59, between "nique" and "The" insert a period Column 2, line 59, change "as" to at Column 3, line 64, change "selaed" to sealed Column 5, line 43, change "10,821" to l0.82l

"5 Claim Should read L Claims On the cover. sheet,-

Column 6, Claims "L and 5"should read 3. and L Column o, line '31, "3" should read 1 Signed and sealed this 21st day of May 1971 Attest:

mum-w LLFLLS'I 3511311, .11, a: MARSHA DArIl-i Attesting Officer- Commissioner of Patents FORM PO-105O (10-69) USCOMM,DC 503754;," 1* u.s. covsnmqzm PRINTING OFFICE: nu o-ass-au 

2. The image screen recited in claim 1 wherein said sodalite has a sodium halide deficiency of from 10 - 20 weight percent.
 3. The image screen recited in claim 1 wherein said sodalite particles have a sodium halide deficiency from stoichiometry of from 10 - 20 weight percent and wherein said image screen has a saturation cathodochromic contrast ratio of at least 20:1.
 4. The image screen recited in claim 3 wherein at least 90 percent of said sodalite particles have a particle size of from 4 - 6 microns in diameter.
 5. In a dark trace cathode ray tube, an image screen comprising a substrate having a layer of cathodochromic sodalite particles thereon of the type wherein the halogen ion is a bromide ion, said sodalite characterized in that the particles are spherical in shape having an average particle size of from 4 - 6 microns in diameter and contain less than the stoichiometric quantity of NaBr, said cathode ray tube having a cathodochromic contrast ratio at saturation of at least 20:1. 